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Ultrafast fluorescence studies of excited-state hydrogen transfer reactions in solution

Toele, P.W.

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Chapter 1

Introduction

Studies of molecular dynamics arc of great interest to chemists because they contribute to

a better understanding of the course of a chemical reaction [1-6]. On a molecular level, chemical

reactivity is often discussed in terms of the free energy of the system as a function of the

generalized reaction coordinate [7, 8]. The temporal evolution of the reaction coordinate is

characteristic of the specific nuclear motions constituting the bond breaking(s) and bond fusion(s)

during the reaction. For chemical reactions in the condensed phase, very often the nature of

solute-solvent interactions influences the functional shape of the free energy or potential energy

surface; in turn, this may be of great influence on the chemical reaction dynamics.

In this thesis we focus on a few molecular systems showing ultrafast intra- and

intermolecular hydrogen transfer dynamics in the condensed phase. Hydrogen atom or proton

transfer reactions are ubiquitous in chemistry and biology [9-13]. Proton/hydrogen atom transfer

reactions may be initiated under various experimental conditions, e.g., by thermal activation [14],

solvent mediation [13, 15] or photoexcitation [16]. To the experimentalist it is a challenge to

study the behavior of the transfer reaction under a variety of different experimental conditions.

Based on the information obtained in the experiments, it is attempted to visualize the functional

characteristics of the potential energy surface and to indicate what reaction pathway is followed

by the system as the reaction takes place.

Hydrogen atom transfer is often used synonymously with proton transfer. The latter

involves a Zwitter-ionic structure and does not require significant electronic redistribution. In the

case of hydrogen atom transfer, the product state is stabilized by electronic and nuclear

redistribution. Quantum mechanical calculations and studies of the reaction in various polar

solvents have been performed to distinguish between both mechanisms [17-19].

Hydrogen atom and proton transfer reactions have been investigated for molecules in

both ground- and excited states. The pioneering work of excited-state hydrogen transfer research

began in the 30-ies [20] and the late 40-ies [21]. All of these studies focused on /«/ennolecular

hydrogen transfer reactions. The first studies of excited-state /'«framolecular hydrogen atom

transfer were reported in the mid 50-ies for the hydrogen atom transfer reaction in photoexcited

methyl salicylate [22],

2 Chapter 1

Studies of hydrogen transfer have been of great significance for the understanding of a

wide variety of elementary reactions [23], For instance, the unique properties of water stem from

the ability of water molecules to form extensive networks through intermolecular hydrogen

bonding [24, 25]. Elaborate calculations and experimental studies resulted in a model in which a

continuous exchange of protons between nearest neighbor molecules is visualized [24]. In studies

of catalysis, hydrogen bonding has often been found to be of vital importance [26], Hydrogen

bonding has been a key for determining the structural specificity of enzymes in restricted

substrates [27] as for instance in membrane proteins [11] or in hydrogenation reactions [28]. For

some excited-state intramolecular proton transfer (ESIPT) reactions, it has been shown that the

ESIPT is driven by excited-state intramolecular charge transfer (ES1CT) [29]. Energy-

stabilization by the polar medium of the possible product species has been found to determine the

branching ratio of several possible ESIPT products. In DNA, proton - and charge transfer are

entangled as well [30]. Proton motion was found to affect long range charge- and electron transfer

in DNA [30, 31]. The main aim of all these studies is to gain insight in the photophysics and

photochemistry of macromolecules that are of vital importance in nature. In addition, proton

transfer studies have also led to a wide range of practical applications through the development of

technological devices such as: solar collectors [32], energy storage systems [33], chemical lasers

[34], high-energy detectors [35] and photolithography [36].

Photoinduced ESIPT can be investigated by employing time-resolved spectroscopy.

Ultrafast laser technology has made it possible to study the ESIPT kinetics in real time with

(sub)picosecond time-resolution [37, 38]. For fluorescent molecules, femtosecond fluorescence

techniques are very powerful. The advantage is here that the temporal behavior of only the

excited-state population is studied, without the interference of bleaching and excited-state

absorption phenomena as in ultrafast transient absorption experiments [39-41].

In this thesis, we are mainly concerned with the excited-state proton transfer dynamics of

a few organic fluorescent molecules of special interest, as detailed below. We focus on

experimentally characterizing the excited-state dynamics of these molecules thereby attempting to

also gain information on the nature of the reaction coordinate and the potential energy surface.

To illustrate a few of the salient features of a photoinduced proton/hydrogen atom transfer

reaction, we make use of the scheme of Figure 1.1. Two minima in the potential energy surfaces

of the ground- and excited-states occur, corresponding to the tautomer forms, before and after the

proton transfer, respectively. The abscis could for instance refer to the normal coordinate of a

skeletal mode that promotes the transfer [42-44]. 'Franck-Condon' photo-excitation of the

Introduction 3

ground state population into the (first) excited-state will produce a non-equilibrated excited-state

population. Subsequently, several population relaxation pathways may occur, as indicated in the

figure by the downward arrows. These pathways include direct radiative (dashed) and

nonradiative decay (wavy) relaxation of the reactant, decay by proton transfer from reactant to

product in the excited-state (rate constant A>T), and finally, radiative (dashed) and nonradiative

decay (curly) from the excited product state to the product ground state, possibly followed by

return to reactant ground state via a proton back-transfer (with rate kPJ). The scheme in Figure

1.1 is illustrative of a single proton transfer process.

0 1

Normalized Reaction Coordinate

Figure 1.1: Schematic representation of a possible photocycle for proton transfer. Photoexcitation leads to

an excited-state proton transfer with reaction rate kPT

In many systems, however, a multi-proton transfer process may take place, for instance in

the case of dimerization of 7-azaindole [45-47], and hydrogen-bridge formation in DNA [48-50].

>.

—

z c

4 Chapter 1

In principle, the scheme of Figure 1.1 then has to be adapted and one has to plot the potential

energy surfaces in multidimensional reaction coordinate space.

In Chapter 3, we study the proton transfer dynamics of the molecule [2,2'-bipyridyl]-3,3'-

diol (BP(OH):), for structure see Figure 1.2. BP(OH)2 has received great interest recently because

it may be applied as a laser dye [51] and because it may be regarded as a model system for a

double proton transfer process in the excited-state (ESIDPT) [52]. In previous studies it was

concluded that photoexcitation of the dienol ground state population into the excited-state,

induces an ESIDPT reaction that is essentially branched [53, 54], see Figure 1.2.

Monoketo

Diketo

Figure 1.2: Scheme oj'the concerted and consecutive ESIDPT in BP(OH):

As illustrated in the figure, the reaction scheme comprises of a concerted and a consecutive

double proton transfer. In Chapter 3 we present the results of our ultrafast fluorescence

polarization spectroscopic study. Using time-resolved polarization techniques we were able to

Introduction 5

investigate the reorientation dynamics of the emissive dipole [55]. The aim of our investigations

is to obtain a better understanding of some of the details of the ESIDPT reaction of BP(OH)2. In

these studies it appeared possible to obtain information about the upper limit (~ 300 fs) for the

lifetime of the fluorescent dienol reactant state. Furthermore, additional support for the existence

of a branched photoreaction was obtained.

In Chapter 4, we present results for [2,2'-bipyridyl]-3,3'-diamine (BP(NH2)2), which is

the diamine analogue of BP(OH)2 and which appears to also show photo-induced double proton

transfer. For BP(NH2)2, the optical properties and excited-state dynamics have not yet been

studied previously. We will show that the photophysical and photochemical properties of

BP(NH2)2 are quite similar to those of BP(OH)2 [56]. After photoexcitation, both compounds give

rise to ESIDPT.

The photophysics of BP(OH)2 en BP(NH2)2 is also of great interest in studies of

supramolecular stacks containing these molecules as functional groups. We present a typical

example of such a supramolecule in Figure 1.3. Synthesized supramolecular architectures have

drawn a lot of interest because they may serve as model systems for natural macromolecules [57],

Synthesized modifications of supramolecules may bridge the gap between synthetic architectures

and biomacromolecules, and have the potential to be suitable to create newly functional systems

[58].

Figure 1.3: Bipyridine based C^symmetrical disk (1) and its subgroup DAC (2)

6 Chapter I

The Cj-syrametrical disk type molecules as presented in Figure 1.3, are able to self-assemble to

stacks of molecules in apolar solvents. The large pheripheral sidechains form solvophobic

domains. Under these conditions of reduced influence of the solvent, interactions between

neighboring molecules may be dominated by intermolecular hydrogen bonding and 71-71

interactions. It has been found that the self-assembling of the discotic molecules gives rise to

helical aggregates [59, 60]. This inherent helicity stems from the non-planarity of the discotic

molecules, which restricts the mobility of the molecule within the aggregate. In polar solvents,

solvation compensates for the secondary interactions and self-assembling of the aggregates is

disrupted. In principle, the photophysics of the single disks or the aggregates can be selectively

investigated by simply changing the polarity of the solvent medium. In Chapter 5, we present the

results of our time-resolved fluorescence study of 1 and its photoactive subgroup 2, in polar and

apolar solutions, applying ultrafast fluorescence spectroscopic thechniques. It will be shown that

the bipyridine functional groups, incorporated into large molecules 1 and 2, still show ESIDPT

[61]. Even in the aggregated state, the Ci-symmetrical disk type molecules exhibit ESIDPT [61].

The studies provide information on the kinetics of the ESIDPT reaction. It will be shown that

significant lengthening of the excited-state lifetime and drastic slowing down of the reorientation

motions in the aggregate, when compared with the single disk, can be specifically attributed to the

aggregated state; these phenomena are found to be representative of a rigid aggregate structure.

In some cases, proton transfer cannot occur directly. When the molecule comprises of

proton -donating and -accepting groups that are unfavorably situated relative to one other, a direct

intramolecular proton transfer does not occur. In such a case, the proton transfer may be

facilitated by a protic solvent, [13, 46, 62, 63] or via hydrogen bonding in dimers [64, 65]. This

type of reaction is then essentially an inte>-molecu\ar proton transfer reaction. Solvent-assisted

intermolecular proton transfer has also been found for aromatic thioketones. Two typical aromatic

thioketones are presented in Figure 1.4.

Benzophyranthione (BPT) Thiocoumarine (TC)

Figure 1.4: Two typical aromatic thioketone: benzophyranthione (BPT) and thiocoumarine (TC)

Introduction 7

These aromatic thioketones were found to exhibit unusual fluorescence from the S2 state and

interesting relaxation dynamics in the S2 state [66-68]. From transient absorption studies in n-

alkane solutions [68-70], it was established that the Si-state relaxation essentially involves

intermolecular quenching by the hydrocarbon solvent. In particular, hydrogen bonding with the

solvent was discussed to be involved in the quenching process. From these studies the reaction

scheme of Figure 1.5 was proposed. The reaction coordinate involves the solute-hydrogen-carbon

(SHC) internuclear coordinate, and long before it has a chance to form solute-hydrogen- and

solvent molecule radicals, the system relaxes via a conical intersection, of the S2 and S\ surfaces

[68], towards the S|-state. This type of reaction is termed 'aborted intermolecular hydrogen

transfer'.

i ! Reaction

Coordinate S + R H S H ' + R'

Figure 1.5: Schematic electronic potential energy surface diagram corresponding with the aborted

hydrogen abstraction from hydrocarbon {RH) in the S2 state of the solute (S)

X Chapter 1

In addition, photophysical studies of thioketones in polar and apolar solvents were performed to

investigate how the nature of the solute-solvent interaction affects the Si-state dynamics [68]. In

the polar solvent acetonitrile. it was concluded that in addition to the aborted intermolecular

hydrogen transfer, exciplex formation with the solvent contributes to the S2 state relaxation [71].

In Chapter 6 we investigate the S2-state photo dynamics for molecule 4H-l-benzopyrane-4-thione

(BPT: Figure 1.4) in the protic solvents water and deuterated water. Of interest is to understand

the mechanism of deactivation of the Si-state of BPT in water and deuterated water. From our

studies we conclude that a hydrogen bonded solute-water complex is involved in the Si-state

deactivation.

To gain additional information on the S2-state dynamics of BPT in solution, we

performed fluorescence polarization experiments (Chapter 7). Our aim was to obtain additional

information on the reorientation dynamics of the transition dipole during the Si-state relaxation.

We examined the orientational anisotropy dynamics in a series of n-alkanes solution to determine

the dependence of the reorientational motions on solvent viscosity. The dynamics for the

orientational anisotropy decay are found to comply with a linear dependence on solvent viscosity.

As discussed throughout in all of the next Chapters, proton transfer is always

accompanied by a variety of additional excited-state relaxation processes. In the condensed phase,

excess energy is dissipated via intra- and intermolecular relaxation processes [72-75], The

relaxation processes include: state-to-state internal conversion [76, 77], intersystem crossing [78,

79], intra- [80-82] and intermolecular vibrational relaxation [83-85] and solvation [86-88]. The

time scales of these processes generally range from tens of femtoseconds to tens of picoseconds.

Thus the investigation of excited-state relaxation, in particular of large molecules, confronts the

experimentalist with a very complicated task indeed. In Chapter 8 we study an example of a

highly fluorescent dye, showing relaxation from an excited electronic state higher than S,. The

structure of this dye molecule, 2,6-diethyl-l,3,5,7-tetramethyl-8-phenyl-4-difluorobora-3a,4a-

diaza-(s)-indacene (BDP) is given in Figure 1.6.

The excited-state dynamics in the first excited-state mainly involves excited-state

relaxation via spontaneous emission [89]. We have investigated the higher excited-state dynamics

of fluorescent dye BDP-1 by means of ultrafast fluorescence spectroscopy measurements. It

appeared possible to determine the Sn -> S, (n=2,3) internal conversion time (typically between

100 and 230 fs) and the ensuing vibrational cooling in the excited S\ state (~ 10-20 ps). BDP-1

represents one of the rare cases in which these relaxation processes could be followed using the

femtosecond fluorescence upconversion technique.

Introduction 9

F I F \ /

BDP BDP-1

Figure 1.6: Scheme of boron-dipyrromelhene (BDP) dyes

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